Try looking at DBPowerAmp, its free and can easily convert any MP3 to whatever you wish and back again, its all run via plugins, and is ready to run from installation
It will easily convert MP3 to WAV, MP2 .. basically if a plugin is available for that format then you can convert to/from that format to whatever format is available as a plugin
I use this for all my conversion needs, plus it will also upload to 99.9% of MP3 players on teh markey, again via plugins
http://www.dbpoweramp.com/
It will happily run in the background as well, so won't hog your entire system lie some so-called conversion software